Saturated Fluorocarbons, Fluorocarbon Hydrides, and Fluorocarbon Halides

1 Fluorocarbons

Two new reviews of the physical properties and associated applications of saturated fluorocarbons have been published. One of these concentrates on outlets for such materials in the electronics industry, while the other seems to have been inspired by current interest in medical and biological circles regarding the use of fluorocarbons and fluorocarbon ethers or amines in blood substitutes (see also p. 98). The use of fluorocarbons as working fluids for heat engines has also been reviewed.

A direct calorimetric determination of the heat of reaction of a poly(carbon monofluoride) of stoicheiometry CF1.12 with fluorine to yield carbon tetrafluoride has enabled the values - 46.7 [plus or minus] 1.0 kcal mol-1 and ~ 115 kcal mol-1 to be calculated for the heat of formation of [CFl.12]n and the C-F bond energy in this material, re~pectively.~ Using this heat of formation and those of polytetrafluoroethylene, perfluoro-n-heptane, hexafluoroethane, and carbon tetrafluoride, it has been found that the heat of formation of 'saturated' carbon fluorides can be expressed by the equation ΔHt°(CFx) = - (44x + 3x2) [plus or minus] 3 kcal mo1- l. The results of an assessment of poly(carbon monofluoride) as a solid lubricant have become available.

More thorough investigation of the 1740 A xenon-sensitized photolysis of perfluorocyclobutane has provided no evidence for the formation of xenon difluoride and shown that the organic products include the fluorocarbons CF4, C2F4, C2F6, C3F6, C3F8, C2F5 • CF:CF,, and trans-CF3•CF:CF•CF3 (the main product) and polytetrafluoroethylene.6 A detailed account has been published of gas-phase pyrolysis of perfluorocyclopropane (-> C2F4 [plus or minus] :CF2), perfluorocyclobutane (->2C2F4), and perfluorocyclohexane (- >C2F4, C3F6, cyclo-C4F6, cyclo-C4F8) in single-pulse shock tubes.

No convincing evidence has been found for insertion of carbon-11 from nuclear recoil reactions into C-F bonds of saturated fluorocarbons, in contrast to the situation with alkanes, for which C-H insertion is the most prominent reaction. Recoil 18F attacks perfluorocycloalkanes to yield products derived from F-for-F substitution, e.g.

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]

(*translational energy, [dagger]vibrational excitation) and, preferentially , C-C fission (l8F* + cyclo-CnF2n -> n-CnF2n18F)

The results of further studies on fluorocarbons, particularly perfluorocyclohexane, as electron scavengers during y-radiolysis of hydrocarbons have been published. γ-Irradiation of solid perfluorocyclohexane at 77 K has been shown by e.s.r. spectroscopy to produce mainly perfluorocyclohexyl radicals ; the spectrum given by perfluoro(methylcyc1ohexane) under the same conditions was not identified, although above 160-170 K the radical [FORMULA NOT REPRODUCIBLE IN ASCII]• was definitely observed. A microwave conductivity detector has been used to follow electron disappearance in pulse-irradiated (2.8 MeV electrons from a Van de Graaff accelerator) gaseous C1 — C4 perfluoro-n-alkanes and related compounds (CF3Cl, CHF3, CH2F2, CH3F, CF3•CH3, C2H5F, C2F4), and radiation damage in orientated polytetrafluoroethylene caused by a pulsed electron beam has been investigated by means of e.s.r. Regarding the effect of radiation on fluorocarbon polymers, it is interesting to note that Mariner IV, in which the thermal shields contained Teflon FEP film and all interconnecting wire and cable was insulated with Teflon, performed flawlessly throughout its 7½ month flight to photograph Mars.

Two interesting variations of Simons' electrochemical fluorination technique for the preparation of fluorocarbon materials are under investigation. One involves electrolysis of solutions of inorganic fluorides in anhydrous organic solvents using nickel anodes [FORMULA NOT REPRODUCIBLE IN ASCII] and the other involves the introduction of gaseous alkanes, alkenes, or alkyl halides (see p. 14) into a porous carbon anode of a medium-temperature fluorine generator (72-95 °C, electrolyte KF,2HF) (e.g. [FORMULA NOT REPRODUCIBLE IN ASCII]). Electrolysis of a solution of potassium bifluoride in acetic acid with platinum electrodes yields no fluorinated product, but similar electrolysis of a solution of naphthalene in acetonitrile containing triethylamine hydrofluoride apparently gives α-fluoronaphthalene (4 — 5 % yield) and other unspecified fluorinated The mechanism shown in Scheme 1 has been proposed to account for the electrochemical fluorination of acetic acid-KHF2 at a nickel anode.lS Kolbe electrolysis of heptafluoro-n-butyric acid has been shown to give perfluoro-n-hexane in ca. 20% yield.

Skeletal rearrangements have been found to occur during the exhaustive fluorination of bicyclo [2,2,2]octane and bicyclo [3,2,1 ]octane at 320 °C with cobalt trifluoride; thus, both these bridged systems yield a mixture of the perfluorobicyclo-octanes (l), (2), and (3) in the ratio of ca. 60:30:10 and cu. 50:40:10, respectively. By contrast, similar fluorination of bicyclo [3,3,0]octane gives only its fluorocarbon counterpart (1). It has been pointed out that these results can be rationalized on the basis of the generation of carbonium ions early on in the reactions via attack of cobalt trifluoride on starting material or intermediates of low fluorine content. Skeletal rearrangements do not appear to occur during CoF3-fluorination of polyfluorobicycIo[2,2,2]octenes (e.g. see p. 9, in keeping with the difficulty of gaining access to cationic species in polyfluorinated systems.

2 Hydrides

Further examination of the complex mixture obtained by vapour-phase fluorination of benzene with cobalt trifluoride at 150 — 200 °C has resulted in the isolation and identification of three hitherto-unknown minor constituents : a 1H,2H,3H,5H-octafluorocyclohexane, 1H,2H,4H/3H-octafluorocyclohexane, and 1H,2H:4H,5H/-hexafluorocyclohexene. As part of this work, all six possible 1H,2H,3H,4H-octafluorocyclohexanes were synthesized via photochemical addition of chlorine to 3H,4H/- and 3H/4H-octafluorocyclohexene followed by reduction of the resultant dichloro-octafluorocyclohexanes with lithium aluminium hydride. Fluorination of benzene at 280 °C with potassium tetrafluorocobaltate(III) [prepared from fluorine and potassium trifluorocobaltate(II)], which is a milder, more selective, fluorinating agent than cobalt trifluoride, has been shown to yield a 1,2,3,4,5,6-hexafluorocyclohexane (probably with the all-trans-1H,3H,5H/ 2H,4H,6H-structure in view of its marked resistance to dehydrofluorination) together with 3,3,6,6-tetrafluorocyclohexa- 1,4-diene, 1H: 3H/4H-hep tafluorocyclohexene, 1H:4H/5N-heptafluorocyclohexene, 1H,2H,4H-heptafluorocyclohexene, 1H,2H:4H/5H-hexafluorocyclohexene, and 1H,2H:4H,5H/hexafluorocyclohexene; only the unsaturated products were obtained by similar fluorination of fluorobenzene or p-difluorobenzene.

Fluorination with cobalt trifluoride of the two Diels-Alder adducts obtained by heating lH-heptafluorocyclohexa-1,3-diene with methyl acrylate yields, inter alia, 1 H-tridecafluorobicyclo [2,2,2]octane, 1H,2H- and 1H,3H-dodecafluorobicyclo [2,2,2]octane, and tetradecafluorobicyclo [2,2,2]octane (4a — d). Dehydrofluorination of the 1H,2H- and 1H,3H-compounds with strong aqueous potassium hydroxide at 100 °C yields only 1H-undecafluorobicyclo[2,2,2]oct-2-ene (5), which gives the 1 H-tridecafluoro-octane (4a) when treated with cobalt trifluoride at 200 °C. As expected, the bridgehead hydride (4a) resists dehydrofluorination as above, and undergoes H-D exchange when treated with a hot solution of potassium hydroxide in deuterium oxide; with methyl-lithium it gives the exceptionally stable tridecafluorobicyclo [2,2,2]oct1-yl-lithium (see p. 86), which yields the parent hydride (4a) and tridecafluoro-1-iodobicyclo [2,2,2]octane (6) when treated with water and iodine, respectively.

The acidity of 1H-tridecafluoro [2,2,2]octane, like that of 1H-undecafluorobicyclo [2,2,1]heptane, can be quoted as evidence against fluorine hyperconjugation as a significant stabilizing phenomenon in fluoroalkyl anions. The conclusion that inductive effects (- I and + Iπ) are fully adequate to rationalize the stabilities of fluorocarbanions is supported by the results of a recent molecular orbital examination of the importance of fluorine hyperconjugation in the 2-fluoroethyl and 2,2,2-trifluoroethyl anions.

Calculations employing the CND0/2 method were performed on a number of geometries of fluoroethane, 2-fluoroethyl anion, 1,l,1 -trifluoroethane, 2,2,2-trifluoroethyl anion, ethane, and ethyl anion and cation; the results are listed in Tables 1 and 2. The energy change between a hydrocarbon and its anion can be taken as a measure of the stability of the anion. Thus, on the CNDO energy scale 2-fluoroethyl anions are about 0.7 eV more stable than the corresponding ethyl anions. The difference between the syn- or anti-2-fluoroethyl anion and the perpendicular isomer, which would be expected not to conjugate, can be interpreted as measuring the amount of hyperconjugation. This difference is 0.1 1 eV, or ~ 15 % of the total energy stabilization conferred by the /βfluorine. Other criteria suggest that the magnitude of this percentage indicates a small role for fluorine hyperconjugation. For example, the calculations indicate that fluorine carries a net formal charge of - 0.35, - 0-37, and - 0.37 in the perpendicular, syn- and anti-2-fluoroethyl anions, respectively. For comparison, hydrogen carries a net charge of – 0•10, – 0•13, and – 0•13, and + 0.17, + 0-24, and + 0-25 in the corresponding unsubstituted ethyl anions and cations, respectively. Thus, fluorine does not appear to delocalize charge by conjugation, even though the CNDO/2 calculations are known to overemphasize charge delocalization from carbanion lone pairs. Comparison of the energy data for the anions CH3•CHB2-, CH2F•CH2-, and CF3CH2- reveals that the effect on stability of introducing β-fluorines is approximately additive. The reduction in energy difference between conformers of trifluoroethyl anion compared with those of 2-fluoroethyl anion, which is coupled with a change in the relative order of conformer stability (see Table l), can be explained by unfavourable 120° conformational interaction.

Perhaps the most revealing information is gained from an examination of the CNDO bond-order parameters (Table 2). If fluorine hyperconjugation were important for the 2-fluoroethyl anion, a large increase relative to fluoroethane in the π-bonding between the carbon atoms might be expected. In fact, for the syn and anti conformations, which are most favourable for π-bonding, a small increase (0.12) in bond order (CND0/2) is noted in moving from fluoroethane to the 2-fluoroethyl anion. It is significant that the magnitude of this increase is similar to that (0.10) observed in the case of ethane-ethyl anion and far less than the case (0.27) of ethane-ethyl cation where hyperconjugation is thought to be important. It appears that fluorine stabilizes negative charge by hyperconjugation little better than hydrogen.

The following pKa values (MSAD scale) have been derived from polarographic data for electrochemical reduction of the corresponding organomercurials (cf. p. 88): [FORMULA NOT REPRODUCIBLE IN ASCII].

Dehydrohalogenation of fluorine-containing mixed halogenoforms and related compounds has been used as a source of fluorocarbenes in a number of investigations reported during the period under review. The results of an investigation of insertion reactions of labelled mono- and di-fluorocarbene with hydrogen halides have been reported; the carbenes, CH18F and CF18F, were generated by secondary decomposition reactions of excited 18F-labelled molecules formed by hot 18F atom attack on various precursors, including CH2F2, CHF3, CF4, and C2F4.

3 Halides

γ-Radiolysis of gaseous trifluoroiodomethane has been shown to yield carbon tetrafluoride, difluorodi-iodomethane, and iodine, mainly, it is believed, via a free-radical sequence initiated by homolytic cleavage of the CF3-I bond. Support for this postulate was obtained from radiolysis experiments involving oxygen and nitric oxide as scavengers for trifluoromethyl radicals, the presence of these additives causing a reduction in the yields of carbon tetrafluoride and difluorodi-iodomethane and leading to the formation of, inter alia, bis(trifluoromethy1) ether and trifluoronitrosomethane, respectively. Kinetic investigations on trifluoromethyl radicals generated by radiolysis of trifluoromethyl halides CF3X (X = C1, Br, or I) in hydrocarbon (reactions studied : hydrogen abstraction from cyclohexane; addition to ethylene) and aqueous [reactions studied: CF3X + e-(aq.) -> CF3. + X-; hydrogen abstraction from formate anion and alcohols; additions to aniline, alkenes, and buta-1 ,3-diene] solutions have also been described.

The rate of combination of trifluoromethyl radicals to form hexafluoroethane has been measured by the flash photolysis of trifluoromethyl iodide coupled with rapid-scan i.r. spectroscopy; in the absence of an inert diluent (Ar, N2, or CO2) carbon tetrafluoride and tetrafluoroethylene were also formed, presumably via fluorine atom abstraction from trifluoroiodomethane by 'hot' trifluoromethyl radicals (cf ref. 27). Photolysis of trifluoroiodomethane has been used in studies on (i) the direction of radical attack on 1,3,3,3-tetrafluoropropene [-> CF3•CHI•CHF•CF3, (75 %) + (CF3)2CH•CHFl (25%); (ii) the rates of hydrogen abstraction from ammonia, ammonia-d3, ethylene oxide, silane, trimethylsilane, tetramethylsilane, and cycloalkanes; and (iii) the competitive addition of the CF3• radical to ethylene and vinylidene fluoride. Radicals formed by photolysis of the fluoroalkyl iodides CF3I, C2F5J, n-C3F7I, (CF3)2 CFI,(CF3)2,CHI, (CF3)2CDI,(CF3)2CClI, and (CF3)2CPhI (the last was synthesized by treatment of CF3•CPh:CF2 with CsF and iodine in DMF) have been trapped with the nitroso-alkane Me3C•NO, and the e.s.r. spectra of the resultant nitroxides have been analysed; relative photolytic stabilities of the iodides were determined by competitive trapping reactions, to give the following order of substituent radical-stabilizing ability: Ph > C1 [??] C2F5 > CF3 > F > H. NN-Difluorodifluoromethylamine has been synthesized in low yield by photolysis of a mixture of difluoroiodomethane and tetrafluorohydrazine.

Re-investigation of the t hermally-initiated addition reactions between trifluoroiodomethane and vinyl fluoride and propene has shown that at 200 °C both olefins yield a mixture of 1 :1 adducts, but in each case the major isomer is derived from trifluoromethyl radical attack on the CH2 group. Bi-directional addition was also observed in thermal reactions between trifluoroiodomethane and trifluoroethylene and hexafluoropropene, the product isomer ratios following closely those found in u.v.-initiated reactions.

Reports in the patent literature on thermal or peroxide-initiated addition reactions between perfluoroalkyl or related iodides and olefinic substrates include a description of a flow system for the continuous specific production of 1:1 ethylene-perfluoroalkyl iodide adducts, e.g. n- C7F15•CH2•CH2I, and the use of these, especially the long-chain variety, in the preparation of intermediates of commercial interest via nucleophilic displacement of iodine has been exemplified.